A metal is a chemical element which can form metal connections and lose electrons to form cations (positive Ion S) (and ionic connections in the case of alkaline). Metals are one of the three groups of elements distinguished by their properties from Ion isation and Chemical bond; the two others are the Métalloïde S and the Non-métaux.
Although metals can be defined by their physicochemical properties (see further), it is also possible to categorize them by employing a characterization of their structure of band: metals have valence bands and conduction which is recovered. This definition makes it possible to integrate into the category of metal metals the polymeric and others made up Organométallique S. It is however not an always sufficient criterion. For example, carbon is not a metal but it has a named phase graphite (it is its phase alpha besides) which corresponds to this definition.
Metals in the periodic tableIn the periodic Table of the elements the diagonal on the basis of the Boron (B) and going until the Polonium (Po) separates the metal elements (in bottom on the left) from the nonmetal elements (in top on the right). The elements placed on this line are Métalloïdes.
Moreover, the metal character of the elements of the same column increases with the number of electrons (i.e. when one goes down in the table). For example, the carbon-diamond (Z=6) is a Isolant, the Silicium (Z=14) is a Semi-conducteur and the tin (Z=50) is a metal.
Properties of metals
Metals are in general solids Cristal flaxes; the mercury is however a notable exception since it is only metal in the liquid state in the normal conditions (25 °C under Atmospheric pressure). Three other metals are liquid around the room temperature: the Cesium, the Gallium and the Rubidium. They are in general malleable and ductile (one can deform them for example by stretching them or by hammering them), they leads well heat and electricity (but not all), and they reflect the light (metallic luster) except when they are covered with oxide (Corrosion). Most of the time, metals are extracted in mineral form more or less crystallized (Cristal) in their ores and almost always combined with one or more other atoms. They are often present at the natural state in the form of Oxyde, in Minerai S: Bauxite for aluminum, Hematite for iron, ilmenite or Rutile for titanium… The ilmenite, for example, principal ore of titanium is a mixed iron and titanium oxide (percentage of two metals between 30 and 70 %). Others can be presented in the form of sulfides Pyrite for iron or of sulfates, even in more complicated form and even in several different forms as one saw for the Fer. Certain metals are present at the not-oxidized state, like the noble metals (gold, platinum) or in rocks of origin meteoritic (Nickel - Fer). They then are called native.
Metals conduct generally well electricity and the heat. At the head money, copper and gold.
Electric conduction in metals can be analyzed in a microscopic or macroscopic way.
From a microscopic point of view, the main reason comes from the metal connection. The metal Atome S form structures 2D or 3D which are repeated, called meshs. Inside, of about free electrons circulate surrounded of the atoms from which they result. It is this electronic movement which is responsible for the good conduction, and more these electrons are free, plus metal is conducting good. These more or less free electrons are called “electrons of conduction”.
From a macroscopic point of view, it is the theory of the energy bands which provide the answer. Indeed, in metals the energy band highest occupied and lowest vacant overlap, or at least are touched. One thus needs little energy to excite a metal and more it is easy to excite it, plus a metal will be able to yield an electron and better conducting it will be.
Magnetic propertiesSome metals present remarkable magnetic properties like the Ferromagnétisme, primarily the Fer, the Cobalt and the Nickel with room temperature. The properties of magnetism vary when one makes Alliage S what can be made profitable to create powerful Aimant S where to cancel the Magnétisme of a metal (iron typically).
Chemical speciationThe various states of oxidation, conformations, complexes or forms transitory represent chemical species distinct from an element and exploit an important role their biodisponibility and their toxicity or ecotoxicity. Certain species of metal elements traces (ÉTM) are more easily assimilable by the organizations than of others, which generates beneficial or harmful effects according to nature and the concentration of metal (essential component or not).
One should not confuse the chemical speciation of an element with its fractionation or its partition. The scientific literature confuses these concepts sometimes what complexes research in these fields.
chemical Speciation: It is the distribution of an element according to various categories of chemical species in a system
- Fractionnement: It is the classification of an element or a group analyzed compared to its physical properties and/or chemical (size of particles, solubility, strength bonding, etc)
- Partition: Distribution of a compound in the various phases of a system (solid, liquid, atmosphere, organic matter, etc) according to specific coefficients of partition
This section thus describes the principal categories of chemical species relating to the ÉTM and present of the chemical examples of species of varied toxic level.
Oxidation and reductionThe state of oxidation or reduction of metals in a system influences their effects on the organizations. For example, chromium oxidized Cr (III) is an essential component (i.e. necessary for the good performance of the organization) and penetrates with difficulty the lipidic membranes of the cells. On the other hand, Cr (VI) proves to be toxic for certain genes, is carcinogenic and penetrates easily in the cells thanks to specific conveyers. Moreover, the most reduced shapes of arsenic are generally most toxic: toxicity of AsH3 > Ace (III) > Ace (V).
Isotopic compositionThe isotopic composition of some elements influences their abundance or their toxicity in the environment. For example, lead comprises a score of Isotopes on the whole and four of them are in a stable form: 204Pb, 206Pb, 207Pb and 208Pb. The 206Pb and 207Pb come from the degradation of uranium and the 208Pb results from the degradation of thorium, two radioactive elements; thus, the abundance of these isotopes increases in time, and the isotopic composition of lead thus evolves/moves according to the stimulated sources of emission. Another interesting example of variation of toxicity is related to the isotopic composition of water (H2O): to replace 60 % of the water of the body of rodents by H218O is without effect whereas a substitution of 30-40 % of this water by of D2O generates the death of these animals.
Composed and complex inorganicMetals are often combined with inorganic ligands to form compounds or inorganic complexes having of the different physicochemical properties. For example, the load, solubility, the coefficient of diffusion or the bonding strength of these compounds influence the transport and consequently the biodisponibility and toxicity of metals in the organizations. For example, certain nickel salts like the chlorides (NiCl2) and the sulfates (NiSO4) are water soluble and of low oral toxicity, whereas the nickel sulfides (Ni3S2) are practically insoluble in water but are carcinogenic.
Composed organicThe organic compounds such sugars, organic acid, made up lipids or others low-weight organics molecular have more or less important affinities with metals. Some of them, of the organic acids like the citric Acid and the Malic acid , contain a functional grouping (hydroxylcarboxyle) which binds easily to metals and which decrease their biodisponibility; these compounds are very studied in terrestrial ecotoxicology because they are excreted by the roots of the plants and the micro-organisms of the ground, creating a synergy which decreases the toxicity of metals in the ground.
Some particular organic compounds as EDTA form very stable complexes with metals which one names chelating. The chelating ones are soluble ligands polydentés slightly acid which form thermodynamically strong complexes chelating-metal; they are sometimes used for the restoration of water and the grounds contaminated with metals or in the chemical analytical methods to extract metals from a matrix.
Organometallic compoundThe compounds Organométalliques contain a connection between carbon and metal. This connection can be of covalent or ionic nature; for example, the connections carbon-sodium and carbon-potassium are strongly ionic, the connections carbon-tin, carbon-lead and carbon-mercury are strongly covalent and the bonds carbon-lithium and carbon magnesium are between the ionic connection and the covalent bond.
For example, the bioalkylation, i.e. the formation of an alkyl (CHx) with a metal by specific micro-organisms, is a frequent process in the grounds and the sediments. However, although the methylation of metals (CH3-métal bond) form of made up rather toxic, certain metal selenium and arsenic alkyls detoxify the metabolism of human and other living organisms. Nevertheless, the majority of the organometallic products resulting from a bioalkylation are of origin anthropogenic, like certain fungicides or products of gasoline combustion, and are very toxic for the central nervous system of certain organizations (like mercury or lead, the tin alkyl derivatives).
Composed or complex macromolecularThe macromolecular compounds or complexes are in extreme cases of representation of the chemical species. They form despite everything a distinct category because they play a particularly important part in the biodisponibility of metals for the living organisms. Indeed, the humic and fulvic acids resulting from the biological breakdown of the organic matter are anions mobilizing the ÉTM contained in the grounds and water. The humic and fulvic acids have very variable and complex structures and a composition but would exploit a significant part the speciation of metals.
Other organic and inorganic particles the such biomass and the colloids adsorb metals and thus decrease their toxicity by reducing their biodisponibility. On the other hand, other anion macromolecules of the living organisms, like certain nucleic acids or the glycosaminoglycanes, bind involuntarily to the ÉTM and cause detrimental mutagenèses for the organization.
Parameters influencing the speciation of metalsThe speciation of metals in the aqueous and solid phases is influenced by several parameters (See also Environnement section of this page):
pH: in general, an acid pH tends to solubilize metals whereas an alkaline pH supports their adsorption;
- organic matter: the organic matter adsorbs metals and is synonymous with stability (not biodisponibility)
- the concentration of the ligands: more the concentrations of the inorganic and organic ligants is high, more metal binds to this ligant and forms a populeuse chemical species
- the force of the ionic or covalent bond: the stronger the bond metal-compound is, the more the chemical species associated with this compound will be stable;
- the Stoichiometry: the stoechiometric principles must obviously be respected to generate the formation of the compounds.
This speciation implies that chemical balance is reached. However, the complexation of metals with the inorganic ligands is very fast because they are numerous in the aqueous phase, but the complexation of metals with the organic ligands requires more time because the sites of adsorption or attachment are less accessible. Consequently, it has been preferable to analyze the speciation of a metal contamination on a stable matrix contaminated for several years that a matrix coldly contaminated with an evolutionary chemical dynamics, without what the analyzes are likely to be skewed.
Moreover, the constant of balance relating to the concept of balance chemical can be illustrated by the reaction: Métaln+ + Ligandn- - > Metal-Ligand The Constante of balance Kéq associated with this equation varies according to the type of bond:
- ionic Bond: ~ 100 < Kéq < 104
- Complex: ~ 104 < Kéq < 108
- Chelating: ~ 108 < Kéq < 1020
Thus, since Kéq is relatively weak for the ionic pairs and higher for the complexes, metals prefer to join in the long run the stable complexes that with the ionic pairs of weaker binding energy.
EnvironmentContrary to the organic compounds, metals are not biodegradable by the micro-organisms. This characteristic generates certain problems of management of the metal contamination. Indeed, the fate of metals in the environment poses analytical big challenges; metals are found in several forms in the ground and water (complex with the organic matter of the ground, minerals, precipitation, ions free, etc) complexing the predictions of toxicity and ecotoxicity.
Toxicity and terrestrial ecotoxicityThe toxicity and the ecotoxicity of metals in the grounds are closely related on their chemical speciation and the concept of biodisponibility; the more mobile the metal species is, the biodisponible it is and the more there is a risk of toxicity on the living organisms. In general, the free metal ions (in solution) thus constitute the chemical form most available for the organizations and most likely to be toxic. However, other species or fractions of metals can be unstable and mobile (fraction unstable or related to free oxides for example) and generate a risk for the organizations.
Moreover, certain metals like iron, copper and zinc constitute essential components to which too weak exposures will involve metabolic deficiencies; however, these metals can also be toxic if the exposure of the organizations exceeds the amount recommended.
Thus, several parameters peuveut to influence the toxicity of metals in the grounds:
pH: generally, an acid pH solubilizes normally motionless metals and thus increases the risk of toxicity
- the composition of the ground: clays and the organic matter of the ground adsorb the contaminants and sequester them in the form of stable complexes slightly mobile, whereas larger particles as sand or the gravel retains less metals of the ground
- the ageing of the contamination: a coldly contaminated site is normally more toxic than a site which undergoes a contamination diffuses for a certain time (slow)
- the level of saturation of the sites of adsorption: the more the sites able to fix metals approach their level of saturation, the more metal will tend to solubilize
AstronomyOne calls metals the products of the reactions which take place in the middle of the star S, which means that, astronomically speaking , all the elements except the Hydrogène are metals. (although under certain conditions of temperature and pressure, hydrogen can have a metal behavior: to see Hydrogen metal)
List principal metals
- money (Ag, noble metal)
- Copper (Cu)
- tin (Sn, of Latin stanium)
- Iron (Fe)
- mercury (Hg, of Latin hydragyrium the Planet Mercury))
- Nickel (Ni)
- Gold (With, noble metal, of Latin aureum)
- Platinum (Pt, noble metal)
- Lead (Pb)
- Titanium (Ti)
- Zinc (Zn)
Many metals are toxic directly or via their compounds, in particular the heavy metals (Plomb, mercury… and well of others and still. the Maximium).
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